July 20, 2009

The sandfish - "like a man diving into water"

My aspiration for the book and this blog, beyond the hope that they would be
read at all, has always been that readers would be surprised. This arose
simply because of the extent to which I surprised myself - clearly, I had a good
idea of the diversity of journeys and connections, and the fascinating
questions, answered and unanswered, that the topic would lead to, but the more I
looked, the more surprised I became. And it doesn't stop. I've lost
track of all the ideas for posts that have simply gone on to the "to do" pile,
shouldered aside by something immediate that comes up. I recently wrote about locomotion
in sand, robotics, the intimate relationships small
critters have with granular materials, and whether sand is a solid
or a fluid or something else entirely. And just a couple of days ago,
browsing through the Science Daily online news headlines, there was a report on
research that intersects with all of these.

I've mentioned Scincus scincus, otherwise known as the sand skink, sandfish, or
sand swimmer, before. This little desert lizard has a remarkable ability to
disappear, and then "swim," beneath the surface of the Saharan sand, rather like
its blind Australian relative, the itjaritjari, the local description
of whose skill is the subtitle to this post. Now new research at the Georgia
Institute of Technology has shed amazing light on how these lizards do
it. The sandfish demonstrates distinctly interdisciplinary skills, and it took
collaboration between the School of Physics and the Interdisciplinary
Bioengineering Program to analyse them. Daniel Goldman and his colleagues built
a small tank filled with glass beads (the sand) in which the density, the
packing, could be controlled by pulses of air from below, a sort of fluidised
bed. They then let a sandfish demonstrate its skills and used high-speed X-ray
imaging to reveal its activities. It was already clear that the shape of the
lizard's body and limbs was well-suited to sand locomotion, and there were some
suggestions that the nano-scale character of its smooth scales also contributed
to its skills. But does it use its limbs to "swim"? The Georgia Tech study
suggests not - that, once below the surface, the sandfish tucks its legs
alongside its body and propels itself with a snake-like wave motion, a
single-period sinusoidal wave that travels from the head to the tail. The news release on
the study discusses the kinematics in more detail, and contains links to several
short videos of Scincus scincus in action; these make for compulsive
viewing, and I've taken the liberty of using screenshots from them to illustrate
this post.

It turned out that the density of the sand made little difference for the
lizard - it simply changed the frequency of its undulation and maintained speeds
of up to six inches a second; its methodology is unique, as Goldman
describes:

The large amplitude waves over the entire body are unlike the kinematics of
other undulatory swimming organisms that are the same size as the sandfish, like
eels, which propagate waves that start with a small amplitude that gets larger
toward the tail....The results demonstrate that burrowing and swimming in
complex media like sand can have intricacy similar to that of movement in air or
water, and that organisms can exploit the solid and fluid-like properties of
these media to move effectively within them

The team then developed an empirical model of the drag forces faced by the
lizard, and, indeed, any object moving through granular material:

Although viscous hydrodynamics can predict swimmingspeed in
fluids such as water, an equivalent theory for granulardrag is not
available. To predict sandfish swimming speed, wedeveloped an
empirical model by measuring granular drag forceon a small cylinder
oriented at different angles relative tothe displacement direction
and summing these forces over theanimal movement profile. The
agreement between model and experimentimplies that the noninertial
swimming occurs in a frictionalfluid.

And, as always with research of this kind, the implications and potential
applications for handling of granular materials go far beyond the skills of a
clever lizard:

In addition to having a biological impact, this study’s results also have
ecological significance, according to Goldman. Understanding the mechanics of
subsurface movement could reveal how the actions of small burrowing organisms
like worms, scorpions, snakes and lizards can transform landscapes by their
burrowing actions. This research may also help engineers build sandfish-like
robots that can travel through complex environments.

“If something nasty was buried in unconsolidated material, such as
rubble, debris or sand, and you wanted to find it, you would need a device that
could scamper on the surface, but also swim underneath the surface,” Goldman
said. “Since our work aims to fundamentally understand how the best animals in
nature move in these complex unstructured environments, it could be very
valuable information for this type of research.”

And, as always with our attempts to better understand the intricacies of
the natural world, it's probably more complex. Research conducted recently in
Germany and also reported in
Science Daily last year, describes specifically how the sandfish uses its limbs
to move "in a way very similar to the crawl stroke in swimming." Ah well, it
just makes the little lizard all the more interesting - watch this space!

Comments

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Michael,

I did not see reference to this in the Georgia Tech news release but since vision is not needed(I would suspect their eyes are protected from the abrasive sand grains by eyelids at these times) when these animals and others like them travel under the sand do you think they must depend almost completely on sound waves for detecting direction and location?

I wonder if they sense subtle temperature gradiations to detect their depth?

Another example of life's fascinating survival adaptations to the environment.